Simple Interest Calculator

Calculate Simple Interest

Estimate interest from principal, annual rate, and elapsed time without compounding.

Principal amount ($)

Annual interest rate (%)

Time period

Interest summary

Simple interest Enter loan or savings terms Interest, total amount, and converted time will appear here.

Simple interest uses only the original principal. It does not compound.

Important Finance Disclaimer

This calculator is for estimates only and educational purposes. It does not constitute financial, investment, loan, tax, legal, accounting, or other professional advice. Always verify important decisions with a qualified professional and the relevant lender, advisor, tax authority, or service provider.

Inputs, units, and periods

Currency inputs are treated as nominal dollar amounts.
The rate is an annual simple interest rate entered as a percent.
Time can be entered in years, months, or days; months are converted to years by dividing by 12 and days by 365.
The calculation uses simple interest and does not compound interest between periods.

Items not included

Origination fees, account fees, taxes, penalties, variable-rate changes, and compounding.
APR disclosures, lender-specific repayment rules, and promotional rate terms.
Inflation or changes in purchasing power over the selected period.

Actual outcomes can differ because of rounding, fees, taxes, insurance, compounding rules, APR disclosures, loan terms, market conditions, provider policies, and the exact timing of payments or cash flows.

What is Simple Interest?

Simple interest is a method of calculating interest where the interest amount is computed only on the original principal amount. Unlike compound interest, simple interest does not take into account accumulated interest from previous periods.

Simple interest is commonly used for short-term loans, car loans, personal loans, and some savings accounts. It's straightforward to calculate and easy to understand.

How Simple Interest Is Calculated

Follow these detailed steps:

Step 1: Gather Your Variables
Identify the principal amount (P), annual interest rate as decimal (r), and time period in years (t). For 6% rate, use 0.06.
Step 2: Apply the Simple Interest Formula
Calculate I = P x r x t. For $5,000 at 4% for 3 years: I = $5,000 x 0.04 x 3 = $600 in interest.
Step 3: Calculate Total Amount
Add interest to principal: A = P + I. In our example: $5,000 + $600 = $5,600 total to be repaid.

Formula

I = P × r × t

Where: I = Interest, P = Principal amount, r = Annual interest rate (decimal), t = Time in years

Total Amount: A = P + I = P(1 + rt)

For months: I = P × r × (months/12)

For days: I = P × r × (days/365)

Example

Personal Loan Example

Problem: You borrow $5,000 at 8% simple interest for 3 years. How much interest will you pay?

Solution:

Principal (P): $5,000
Rate (r): 8% = 0.08
Time (t): 3 years
Interest: $5,000 × 0.08 × 3 = $1,200
Total Amount: $5,000 + $1,200 = $6,200

Quick Calculation Tips

Simple interest is always LESS than compound interest for the same rate and time
Most auto loans use simple interest - you pay less if you pay early
Convert months to years: 6 months = 0.5 years, 18 months = 1.5 years
Simple interest favors borrowers; compound interest favors savers/investors

Common Mistakes to Avoid

Not converting time to years
The formula requires time in years. 6 months must be entered as 0.5, not 6.
Using rate as percentage
Enter 5% as 0.05, not 5. The decimal form is required for the formula.

Frequently Asked Questions

What's the difference between simple and compound interest?

Simple interest is calculated only on the principal amount. Compound interest is calculated on the principal plus any accumulated interest. Over time, compound interest results in higher returns for investments and higher costs for loans.

When is simple interest used?

Simple interest is typically used for short-term loans (less than a year), car loans, some personal loans, and bonds. Most mortgages, savings accounts, and long-term investments use compound interest instead.

How do I convert months or days to years?

For simple interest calculations, divide the number of months by 12 to get years (6 months = 0.5 years). For days, divide by 365 (or 360 for some financial calculations). For example, 90 days = 90/365 ≈ 0.247 years.